Nonlinear optical (NLO) materials have been used in electro-optic devices for more efficient processing and transmitting of information in the field of fiber optic communications. The NLO materials used in these devices have in general been inorganic crystals such as lithium niobate (LiNbO.sub.3) or potassium dihydrogen phosphate (KDP). More recently, nonlinear optical materials based on organic molecules, and in particular polar aromatic organic molecules, have been developed.
Organic nonlinear optical materials have a number of potential advantages over inorganic materials. Specifically, they have fast response times, small dielectric constants, good linear optical properties, large nonlinear optical susceptibilities, and high damage thresholds. In addition, organic materials can be easily fabricated into integrated device structures when used in polymer form. Organic crystals of 2-methyl-4-nitroaniline have been shown to have a higher nonlinear optical activity than that of LiNbO.sub.3.
There are various known polymeric organic materials which possess specific nonlinear optical properties and various known processes for making such polymeric organic materials. Many of the current polymeric organic materials prepared by the prior art are prepared by blending a NLO molecule into a polymer host material. "Blending" herein means a combination or mixture of materials without significant reaction between specific components.
EP 218,938 and U.S. Pat. No. 4,859,876 have used an approach of incorporating NLO active molecules into amorphous polymer host matrices for NLO media. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of molecules which have NLO activity including azo dyes such as Disperse Red 1. It is known that an NLO active material such as azo dye Disperse Red 1, (4, -[N-ethyl-N-(2-hydroxyethyl]amino-4-nitro azobenzene), may be incorporated into a host by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described in Applied Physics Letters 49(5), 4 (1986) and U.S. Pat. No. 4,859,876.
While the doped polymer approach offers some advantages over organic and inorganic crystals, the approach has a number of problems. First, the stability of the NLO activity over time for such materials has been shown to be poor. A problem associated with a polymer with NLO properties produced by simply blending NLO molecules into a host polymer is that these polymer materials lack orientational stability. There is significant molecular relaxation or reorientation within a short period of time resulting in a loss of NLO properties. For example, as reported by Hampsch et al., Macromolecules 1988, 21, 528-350, the NLO activity of a polymer with NLO molecules blended therein decreases dramatically over a period of days at room temperature.
In addition, the NLO dopants in the blended polymeric media plasticize the polymer host matrix, lowering the polymer glass transition temperature (Tg). Lowering the polymer T.sub.g has the effect of lowering the temperature stability of the electrically oriented NLO material or NLO medium. Near the Tg, segments of the polymer become mobile and the NLO active dopant molecules which are oriented electrically undergo orientational relaxation. Once orientational relaxation has occurred, the NLO medium exhibits no NLO activity.
A third problem with the doped polymers is the poor solubility of the NLO chromophore in the host matrix. This limits the concentration of NLO properties that can be incorporated in the polymer matrix. Finally, the NLO chromophores tend to aggregate at relatively low doping levels (e.g. 5-20 percent w/v). Such aggregates scatter light and reduce the transparency of the waveguides to unacceptable levels.
Another disadvantage is that the polymer employed may have a low glass transition temperature, or lack sufficient tensile strength or other desirable properties for optical devices.
Generally, the incorporation of molecular structures which have NLO activity into the backbone of a polymer chain will decrease the likelihood of the structural reorganization in comparison with polymers in which the NLO active molecule is simply blended. It is, therefore, desirable to provide a polymer material with NLO groups covalently bonded to the backbone of the polymer material to minimize relaxation effects.
Amine curing agents have long been used as curing agents for epoxy resins. Amine curing agents are discussed in Lee and Neville, Handbook of Epoxy Resins, McGraw Hill (1967), pages 8-1 to 8-18 and 9-1 to 9-15. Amine curing agents are also discussed in U.S. Pat. Nos. 4,330,659 4,814,414and 4,822,832.
Japanese laid open publication Nos. J-63-275,553 and J-62-210,431 disclose various organic nonlinear optical compounds containing hydrazone functionalities which are useful for NLO applications. Specifically, J-62-210,431 discloses nonlinear optical materials containing nonlinear optical hydrazones as powders, molecule inclusions within the host lattice, thin layers deposited upon carriers such as films, monocrystals, and solutions. The hydrazones of J-62-210,431 may be bonded in the form of a pendant group to a polymer such as a polydiacetylene.
It is an object of the present invention to provide nonlinear optical aminoaryl hydrazones as curing agents for epoxy resins, and as suitable monomers for polymeric compositions such as poly(amino ethers), polyimides, polyamides, and polyureas.
It is further the object of the present invention to provide epoxy polymers or epoxy based polymers containing covalently bonded aminoaryl hydrazone moieties in the structure of the polymers exhibiting enhanced nonlinear optical activity and stability.
It is an additional object of the present invention that the polymers comprising the NLO materials have relatively high glass transition temperatures. A high glass transition temperature will correlate with high temperature stability of the NLO material or medium.